Radio receiver



Fb. 5i 947. Y E. LAK-m 2,416,328 l RADIO RECEIVER Filed July 5, l942 mvENToR 15M/Lf A5/Al ATTO EY Patented Feb. 25, 1947 RADIO yRECEIVER Emile Labin, New York, N. Y., assigner to. Federal Telephone and Radio Corporation, a corpora.-

. tion of Delaware .l Application July' `3, 1942; Serial No. 449,595'

This invention relates to. receiverv systems and more particularly to timemodulated impulse re,- ceivers;

For the.v purpose of economy in power and for' preservation of a moolicum of secrecy transmis'- sion of energy' by time modulatedv impulsesv has been resorted to as described' in United States. patents to A Reeves, Nos. 2,266,401 and 2,2565336 and United Statespatenti to E. Deloraine and A. Reeves, No. 2,262g838'. A particularly. valuable. type ci. suchA systems from the standpointr of avoiding interception of thefmessages is thatI-d'escribed in my copending application entitled Pulse modulation system, Serial No. 386,282; filed April 1, 1941, and in the eopending applica-` tion of E. Deloraine and myself' entitled Pulse transmission system, Serial No. 425,108, l'ed December 3l, 1941. In accordancewith this proposed system successive-pairs of impulses areuse'd respect to said constant timing limit. The time displacement of the impulses, however,v is very small compared' to the timeintervalbetween successive impulses so that the period defined' by either odd or even impulses may appear substantially constant. Y

A receiver may then be used in which is provided a selective circuit tuned substantially to a harmonic of the period. The harmonic is s pulses of a. pair at one extreme position of modulation, such as at the constant` Vtiming limit of the `successive impulses, is an odd number of half wavelengths whereupon zero` energy is passed" by the filter. At the other extremeposition of modujfl lation, the interval spacing between impulses oifA a pair is such that considerable energy'is passed' by the lter. At the normal unmodulated posi-ir tion of the impulses, energy4v at a level intermediate'the two extremes, passes the lter. Thus,- yis possible' by simplefilteri'ng and detecting,'it to receive the signals.

' In order to receive the signals, a harmonic which will' work properly with the given interval of the .impulse pairs and the periody between the pairs must be chosen.v If the normal impulse y spacing is chosen-,sathat the spacings between,

Vchosen that the interval spacing between` imimpulses off-the pairs and between thev pairsofii-mpulses approachwequality, any! odd harmonie be capable oi useior detecting thesis-nais., y

I-n practicing this sys-tem a; relatively high; harmonic is generally used. The energyflevel of. a high harmonic: is generally rather low remitiring very sensitive detecting equipment.. Further more, the signal to interference; ratio may be. rather low due toltheflow signal amplitude.

It is an object oi? myfinventionlto providea` receiver of; the general type outlined abovewhich is relatively freeffrom the ,disadvantages outlined.

It isv a further object, er my invention toprovide a'. receiver for time modulated impulse .tra-ins. which is; relatively free; from interferencerdisturmances. Y i v ItV is a' still further; object: of invention, to prov-ide a receiver for time modulated. impulser trains which does' `notreciuire 'such sensitive-s de-V tecting apparatus as was necessary in earlier proposed? receivers. v

Accordi-ng tov a ffeatinez-iof my invention, the` i objects are accomplished; byt providing means for regenerating or reshaping thezreceivedimpul'sesf before' passing? the: energyv through; a selective@ band-pass filter; 'Ihisis` done-.byusingthe re.- ceived4 impulse to` initiate; a new pulse withq-.oneedge thereof, and this new pulse is made of apare-l determined 'durationfindepend'ent of the signal level. Moreover, the new impulses are preferably made of aI predetermined substantiallyv constant:Y

amplitude level.

According to a further feature of my invention;v

A better understanding oi my inventionrand" the objects and featuresfthereof may beT had 'from the particular description made with referencev toL the accompanying drawing' in which:` '1- Fig. 1 is a set of` curves used" to: explain'the feature of my invention;

Fig. 2 is a block diagram of a simplified receive'rf' made in accordance with myinventiomiand Fig. 3 shows a more detailed circuit diagram-1 of a simpleform of shaping circuit inl accordance with my invention. y f i v Turning iivrstto' Fig'. 1, a brief explanation of* the operation of the transmitter system describedf more fully in my abovevreierr'ed' to application,`

Serial No. 386,282 will be given. The top curve.

ing three pairs, |-`-2, 3*-4 and 5 6. vImpulsesV l-2 and 3--4 areshown positioned most close y v while impulses 5-6 represent the highest positive The details of this type of modulation signal. system are completely described in my above-1 identied application and need not be repeated here since the present invention the receiving system.

relates only to The next lower curve A represents the impulses y as they are received at a receivingfpoint, vItwill be noted that impulses la and 2a form a pair in4 which the spacing is at vits closest position l'while it is seenthat each pair `of impulses is repeated(l at a substantially constant period.

, Since the impulses representing the time displaced modulation are either both moved toward each other from the normal reference axis or are both moved away from each other simultaneously, :the signal may be translated by the use of a properly chosen harmonic frequency. vIt is, therefore, possible with this type of system to receive the signals fby means of a sharply tuned filter which will pass only the particular related harmonics and the modulation signals immediately adjacent thereto. In such systems, how-v ever,` a relatively high' harmonic is preferably used'in order that the energy level may change substantially for slight impulse displacement; The amount of energy of such high harmonic in the received signal is relatively feeble and accordingly requires a more sensitive amplifier than is required to detect'the entire impulse. Accordingly, I provide means for reshaping and enlarging the received impulses without' disturbing their normal spacingprior to the harmonic selection so that stronger signal indications may be ob-` tained.l A Y,

'This feature'of my 'invention may be more readily understood by a further reference vto Fig.1

l lof the drawing. The leading edge of each of the; impulses la to 6a inclusive as they areY received causes the shaping circuit to be moved into operative position.- The circuit then preferably re'-` mains in this operative position for a predetermined length of/time when it again returns to normal unoperated position, thus producing square wave impulses such as shown at Ib, 2b, etcQ o1' curve B. In the example illustrated, it can then be seen that if harmonic curve C is considfered, impulses I and 2 will produce the impulses Ib and 2b which intersect the portions Ic and 2c of harmonic curveC. It is clear that in this posi tion impulses Ib and 2b intersect parts Ic and 2c of equalY length and opposite-sign-so that at the receiverthezero indication is obtained. This, of course,.means that thereis no signal energy at `the harmonic frequency in this position. Impulses 3b and 4b are similarly positioned to intercept portion 3c and 4c of curve C. In the other f extreme modulation position of the impulses (as represented by 5a-6a), square wave 517-61) vwilll beproduced which intercept the portions 5c--6c of curve C. In this position-itis seen that. al con- '4 siderable energy component of the harmonic frequency will be derived from the waves 5in- 6b. At the normal unmodulated position the impulses will be positioned midway between those shown circuit in accordancewith my invention. The

incoming high frequency impulses are impressed on the input of the radio frequency and intermediate frequency amplifier 2U. This amplifier preferably includes la first detectingv means in order'that extremely high radio frequencies may be reduced to a lower value. If .the impulses are also amplitude modulated to convey a further message, these amplitude modulating signals may be detected in an amplitude detector 22. and impressed on `earphones 23 or with proper amplification on to any form of sound Itranslater.

Preferably, the time modulated impulses are passed" from amplifier 20 to a clipper amplitude limiter 2I. VSuch a limiter serves to clip off the lower part of the 'impulses where they tend to trail out reducing noise effectsand to'limit the amplitudes of the pulses to a constant level. It is clear that the clipping eiect is the more importantof the two so far as reduction of noise level is concerned. This amplitude limiter is especially desirable if the impulses have been amplitudemodulated but is often desirable in any case since itrmay serve to cut off theparts of the impulses where the greatest-noise level may be present.`

The limited impulses are .then passed on to a shaping circuit 24 which imparts to these 'im-r pulses the rgeneral form'shown in curve B ofl Fig. 1. VThis shaping circuit will be described in rmore detail later, vIn order that impulses Ib, 2b,

etc. may be given the square topped form, I pref'-,

erably provide a further amplitude limiter 25.

The limiter may be omitted, in which case thek is clear, however, that with suitable amplification any type of translator may be used.

Impulses Ib, 2b, etc. are preferably'made suflll ciently wide to encompass substantially the entire scope of signals la and 2 a, so that thel Yending of the square waves ls not iniluenced bythe trail-lf, ing ends of the incoming pulses.. If this is not;

convenient, ysome other precautions should. be

taken to render the time duration of the square waves delivered from .the Shaper independent of;v the signal applied to the input thereof. This is.` avery important feature of the invention; 'andy whether the-leadingor trailing `edge -of the arriv ing pulses be made use of to determine the time- {of'starting the square wave, the circuit should inj either case be arranged sothatthe duration of zthese square waves wlllbe independent of the duration oramplitude of the input signal. A

The abovefeature has the veryimportant gid/g., vantage of substantially reducing :the: noise-tof.)

signal ratio. This may be most easily under'- stood if it is noted that the previously known.

types of receivers (such "as those disclosed in U. S. Patents 2,266,401 and 2,262,838 above 'mentioned or in copending U. Si applications Serial No. 425,108, filed December 31, 1941, by E". M. Deloraine and E. Labin for Pulse transmission systems and Serial No. 386,282 flied April l, 1941, by Emile Labin for Pulse modulation systems) were inherently more or less responsive to variations in the width of the received pulses as well as being responsive to variations in the time po sition thereof. l

By the limiting and clipping of Athe received pulses these are converted into trapezoidal pulses with iiat tops and with flat valleys between so that substantially the only influence of noise voltages of moderate strength is to very slightly vary the time positions of the sloping front and rear edges ofthe trapezoids. front and rear edges could be made absolutely vertical, noise voltages could not displace'the time positions of these edges. In practice, however, the slope of these edges cannot exceed a finite maximum depending upon the band width of the receiver. Noise voltages therefore are capable of slightly shifting the time positions of these sloping edges. One effect of these shifts of edges backward and forward in respect to time is that the width or duration of the trapezoid is varied, while another effect is that the mean position of. the trapezoid. in time is varied. In view of the fact that theprior art types of receivers responded to both of these effects the response of these receivers to noise was very considerable. By deriving from the trapezoidal pulses waves having a constant duration and having a shape determined by say the leading edge of the arriving pulses the present invention eliminates that part of the noise effect due to variations in the width of lthe pulses.

It Should be noted moreover that impulses such as shown at la and 2a, etc. of Fig. 1 are generally much steeper on their leading edges and tend to trail oii on their trailing edges. For this reason the shaping network if controlled by the leading edges (as is preferred) tends to further reduce the noise-tosignal ratio which might otherwise exist. If, for any reason, control by the trailing edge is desired, the impulses may be passed through a differentiating circuit and a clipper before application to the multivibrator.

It should further be noted that for strongest signal production impulses lc, 2c, etc. should be i It is clear that if these cathode of tube VI just sufficient to maintain the bias of the grid of tube VI at cut-01T.

When an impulse arrives over a coupling condenser 3l, a positive voltage developed across resistance 32 is applied to the grid of tube VI causing this tube to become conductive. Tube VI immediately applies a negative potential over a coupling condenser 33 to the grid of tube V2 causing this tube to be biased below cut-off and drivingv the vcathode offtube VI negative so that the grid is further positive relative thereto. Accordingly, upon arrival of an impulse the ultimate positive valueV of signal is` almost immediately arrived at. The output froml tube vVI causes a voltage drop across resistance 36 which is passed on to the band-pass filter 26' preferably via limiter 25 (Fig. 2). After the first impulse passes the charge on condenser 33leaks olf through a resistor 34 until V2 again becomesfconductive and the current through V-I is` abruptly entirely cut off. The'time constant of the circuit including condenser 33 and resistor 34 determines the duration of the operative condition of the shaping circuit 24 and asa consequence the duration of a pulse. This time constant may be changed by adjusting resistor 3!! to the de@ sired value. The adjustment 'of resistor 30may serve to vary the cut-01T points of the tubes VI and V2 and thus to adjust the duration of the impulse without changing the time constants of circuits t3. 3i; The square wave impulses pro-'f duced in this multivibrator'circuit 24 are taken oi' across grid cathode resistor 35 and applied'to the input of the band-pass filter 26 of Fig. 2.

It is clear that other forms of shaping circuits may be designed than the simple multivibratorI circuit shown herein. The prime requisites of my' invention aretnat the shaping networkV be` such that it will be brought into operation by one of the edges of the received impulses and will be made `to continue in operation for a period of substantially iixed length. It is further clear that if desired, a receiving circuit shown in Fig. 2 mayk be provided with other apparatus for blocking out all signals between the times when the impulses are to be received as disclosed in my above referred copending application.

Many other modifications of my invention may occur to those skilled in the art and it is considered that all of these obvious modifications of rnv invention are included in the scope thereof.

What is claimed is:

1. A receiver for detecting time modulated impulse energy wherein the impulses are modulated by pairs at least one of the pulses of each pair being time displaced relative to the other between two limits according to the instantaneous value of the modulating signal, one of such limits being defined by a constant timing interval; said receiver comprising means for shaping the received impulses into substantially constant duration impulses, filter means passing substantially only a band of frequencies immediately about a harmonic of said constant timing interval, whereby energy of said constant duration impulses is passed according to the time displacement thereof from one of said limits, and means to detect the energy passed by said lter means.

2. A receiver according to claim 1 wherein said shaping means comprises means triggered by the leading edge of each of said received impulses for producing said constant duration impulses.

3. A receiver according to claim 1 wherein said shaping means, comprises means triggered by the leading edge of each of said received impulses for producing said constant duration impulses and means for limiting the impulses to a subtion in one direction by the leading edge of each i received impulse and to return to said normal condition a predetermined time after movement in said one direction. f

`5. A receiver according to claim 1 wherein the shaping means is adjustable for obtaining output impulses of a duration equal to a half periodi of the harmonic of said constant timing-interval.

6. A receiver for segregating and detecting time modulated impulse energy wherein 'the impulses are modulated bypairs at least one of the pulses of each pair being time displaced relative to the other between two limits according to the instantaneous value of the modulating signal, one of such limits being characterized by a constant timing with respect to pulse pairs; said receiver comprising amplitude limiting means for limiting the received impulses to' a predetermined level to remove any amplitude modulation effects therefrom, means for shaping the amplitude limited impulses into substantially constant duration im- Y pulses, and lter means passing substantially only a band of frequencies close about a harmonic-of said constant timing characteristic of the pulse pairs 'whereby energy of said constant duration impulsesis passed according to the time displace- 'ment thereof from one of said limits, and means being time displaced relative to the other between two limits according to the instantaneous value of the Vmodulating signal, one of such limits be ceived impulse.-Av

ing'characterized by a'constant timing with re-,

spect to the pulse pairs; which method comprises,

shaping the impulses into substantially constantduration impulses and passing energy of the constant duration impulses in amounts accordingvto the time displacement thereof with respect to a;

harmonic of the constant timing characteristic-ofi the pulse pairs. e

f 8. The method defined in claim 7 wherein duration of said constant duration -impulsesis- `10. The method defined in claim 7whereinvthev shaping operation is characterized by timing the; leading edges of each constant duration impulse with the leading edge of the corresponding received impulses, and the duration of said constant duration impulses is selected equal to a half peri od of said harmonic. y Y

EMILE LABIN.-

REFERENCES CITED The following references are of record in the file of this patent:

Y UNITED STATES PATENTS Number' Name Date i 2,113,214 Luck ADI. 5, 1938 2,266,526 White DBC. 16, 1941 

